Solar Energy: What To Do When the Sun Sets

Solar-thermal plants use mirrors or "heliostats" to focus sunlight on a tower receptor that produces steam to generate electricity.

A major barrier for solar power has always been that it doesn’t work at night (Duh). A few years ago, developers of big “utility-scale” solar projects were able to shrug this off to some degree. But Oakland-based BrightSource Energy has reversed field and decided to add to several projects the ability to store electricity for distribution after dark.

BrightSource managers say times have changed. Where utilities once wanted all the renewable capacity they could get, to meet state requirements, the priority has since shifted to having those renewable electrons available when they’re needed.

“The challenges of integrating photovoltaics and wind into the grid have driven a much deeper appreciation for those that can be highly reliable,” BrightSource CEO John Woolard told me in a phone interview.

But another driver is — well — us. When I interviewed Woolard a couple of years ago, I asked him why his company wasn’t including storage technology in its California projects. He said it wasn’t needed in California, which had a different pattern of electrical use than, say, Arizona.

That’s changing. Woolard says peak demand, which has traditionally hit around 4 p.m., has been shifting to later in the day, and by the end of this decade, will probably happen around 6 p.m. He says changing lifestyles are behind the shift, such as when people arrive home and fire up their air conditioners and other appliances.

BrightSource says it will use a molten-salt technology to store the power, rather than huge banks of batteries or experimental technologies such as flywheels. “That’s a solution for 2050 or 2060,” Woolard told me, “depending on whether you’re an optimist or a pessimist.”

“The molten salt, with properties like water at temperatures above its 240oC (464oF) melting point, is pumped from a large storage tank to the receiver, where it is heated in tubes to temperatures of 565oC (1049oF). The salt is then returned to a second large storage tank, where it remains until needed by the utility for power generation. At that time, the salt is pumped through a steam generator to produce the steam to power a conventional, high-efficiency steam turbine to produce electricity. The salt at 285oC (545oF) then returns to the first storage tank to be used in the cycle again.”

BrightSource will add molten-salt units to three of its projects in Riverside and San Bernardino Counties, but not to its Ivanpah Valley project, already under construction near the Nevada border. Spokesman Keely Wachs says that the added cost will be “fairly nominal,” and that expanding the plants’ operating hours will, in effect, reduce the price of energy from those plants.

CEO Woolard says that despite plunging prices for conventional photovoltaic solar panels, BrightSource will not be joining a trend among developers to convert some of their giant solar arrays to PV. He says PV output tends to peak at noon (PV power is a function of light, not heat), so their output is falling just as demand is rising. He says that by adding storage capacity to solar-thermal plants such as his,”You can extend when you deliver power and and you’re delivering more of it when the real system peak is.”

About the author

Craig is KQED's science editor, specializing in weather, climate, water & energy issues, with a little seismology thrown in just to shake things up. Prior to his current position, he launched and led the station's award-winning multimedia project, Climate Watch. Craig is also an accomplished writer/producer of television documentaries, with a focus on natural resource issues. View all posts by Craig Miller →

MORE POSTS ABOUT

What will it really take to meet the state’s aggressive carbon reduction goals? As the centerpiece of California’s climate strategy, the law known as AB 32 gets all the attention. But a little-known component of the state’s plan to mitigate … Read More

How many gallons to run that microwave? We hear a lot about how green our energy is in California. Instead of using coal, the state runs on natural gas and increasingly, renewable power. But there’s a hidden cost to our … Read More

Remote deserts would seem to be the ideal place for Big Solar — were it only that simple Can threatened tortoises and utility-scale solar plants coexist in the California desert? Since the solar rush began a few years ago, results … Read More

How many gallons to run that microwave? We hear a lot about how green our energy is in California. Instead of using coal, the state runs on natural gas and increasingly, renewable power. But there’s a hidden cost to our … Read More

We’ve Moved!

About Climate Watch

Climate Watch is KQED's multi-year initiative to provide in-depth coverage of climate-related science and policy issues, with a specific focus on California. More …

Water and Power

Think water and electricity don't mix? Here in California, we wouldn't have one without the other. Now climate change is threatening to alter our water supply, and the impact could show up in your electric bill. Explore the series.